Implantable cardioverter-defibrillators (ICDs) and other invasive anti-arrhythmic devices are well known in the art. A typical ICD includes a sensing circuit, which detects the onset of fibrillation or other potentially dangerous arrhythmic activity. When fibrillation is detected, the ICD applies a short, high-intensity electrical shock to electrodes in or on the heart, causing current to flow through the heart. The shock typically delivers about 10 joules of energy. The object of the shock is to terminate temporarily all cardiac activity, following which rhythmic activity of the heart resumes, either spontaneously or due to application of pacing pulses. In a conscious patient, the shock of defibrillation is very painful. It is noted that if a high level of the applied energy is concentrated in a relatively small region of the heart, then transient or permanent injury may be caused to the affected region.
Cardioversion, in which a shock of much lower intensity is applied to the heart, may also be used to interrupt arrhythmic activity. The cardioversion pulse attempts to interrupt arrhythmia by correcting the synchronization of contraction of different parts of the heart muscle. Cardioversion can be useful in terminating atrial fibrillation or ventricular tachycardia, but it is generally not effective in controlling the more serious condition of ventricular fibrillation.
U.S. Pat. Nos. 5,797,967 to KenKnight and 5,472,453 to Alt, which are incorporated herein by reference, describe methods and apparatus for applying electrical energy to the heart so as to treat arrhythmias.